One of the most critical aspects of an uninterruptible power supply (UPS) is its standby or back-up battery system, where super capacitors are now beginning to play a role.
A super capacitor resembles a regular capacitor except that it offers high capacitance in a small package. Energy storage is by means of static charge rather than an electro-chemical process, inherent in lead-acid uninterruptible power supply batteries. Applying a voltage differential on the positive and negative plates charges the super capacitor (this concept is similar to an electrical charge that builds up when walking on a carpet).
Their design makes them ideal for small uninterruptible power supply installations whereby they are being utilised in favour of a battery set or to reduce the potential for battery discharge during momentary mains power failures.
The amount of energy that can be stored depends upon the active material used in the design of a super capacitor. Potentially, it can achieve up to 30kW of stored energy.
A super capacitor (also referred to as electric double layer capacitor, electrochemical double layer capacitor or ultra capacitor) consists of two electrodes constructed from a highly activated carbon material, which may be woven. Whereas a regular capacitor consists of conductive foils and a dry separator, the super capacitor crosses into battery technology by using special electrodes and some electrolyte. There are three types of electrode materials suitable for the super capacitor: high surface area activated carbons, metal oxide and conducting polymers. The high surface electrode material, also called Double Layer Capacitor (DLC), is least costly to manufacture and is the most common. It stores the energy in the double layer formed near the carbon electrode surface.
The carbon activated electrodes provide a large reticulated area upon which an active material such as Ruthenium Oxide is deposited. The material provides an enormous area, for example, 1000 square meters per gram of material used. Cellulose paper with polymeric fibers to provide reinforcement is typically used as the separator between the electrodes. Electrolyte is usually diluted Sulphuric Acid. Ruthenium Oxide is converted into Ruthenium Hydroxide by a chemical reaction and this enables energy to be stored.
To operate at higher voltages, super capacitors are connected in series. On a string of more than three capacitors, voltage balancing is required to prevent any cell from reaching over-voltage.
Energy within a super capacitor is quickly available – and this is one of its greatest advantages. When coupled to an existing battery set, they can inhibit battery cycling for momentary interruptions, which helps extend the working life of the set. A super capacitor’s working life is typically ten years (double that of an average UPS battery). They can also operate over a wide temperature range (minus 30 to 45 degrees centigrade).
o Virtually unlimited cycle life – can be cycled millions of times.
o Low impedance – enhances load handling when put in paralleled with a battery.
o Rapid charging -super capacitors charge in seconds.
o Simple charge methods – no full-charge detection is needed; no danger of overcharge.
o Linear discharge voltage prevents use of the full energy spectrum.
o Low energy density – typically holds one-fifth to one-tenth the energy of an electrochemical battery.
o Cells have low voltages – serial connections are needed to obtain higher voltages. Voltage balancing is required if more than three capacitors are connected in series.
o High self-discharge – the rate is considerably higher than that of an electrochemical battery.
Whereas the electro-chemical battery delivers a steady voltage in the usable energy spectrum, the voltage of the super capacitor is linear and drops evenly from full voltage to zero volts. Because of this, it is unable to deliver the full charge. If, for example, a 6V battery is allowed to discharge to 4.5V before the equipment cuts off, the super capacitor reaches that threshold within the first quarter of the discharge cycle. The remaining energy slips into an unusable voltage range. A DC-to-DC converter could correct this problem but such a regulator would add costs and introduce a 10 to 15 percent efficiency loss.
The charge time of a Electric Double Layer Capacitors is about 10 seconds. The ability to absorb energy is, to a large extent, limited by the size of the charger. The charge characteristics are similar to those of an electrochemical battery. The initial charge is very rapid; the topping charge takes extra time. Provision must be made to limit current when charging an empty super capacitor.
In terms of charging method, the super capacitor resembles the lead-acid battery. Full charge occurs when a set voltage limit is reached. But unlike the electrochemical battery, the super capacitor does not require a full-charge detection circuit. Super capacitors take as much energy as needed. When full, they stop accepting charge. There is no danger of overcharge or ‘memory’.
Electric Double Layer Capacitors are relatively expensive in terms of cost per watt. Some design engineers argue that the money would be better spent providing a larger battery by adding extra cells. But the super capacitor and chemical battery are not necessarily in competition. They enhance one another.